{"id":"CVE-2020-6817","aliases":["GHSA-vqhp-cxgc-6wmm","PYSEC-2020-340","SNYK-PYTHON-BLEACH-561754"],"url":"https://o3.security/vulnerability/CVE-2020-6817","summary":"regular expression denial-of-service (ReDoS) in Bleach","details":"### Impact\n\n`bleach.clean` behavior parsing style attributes could result in a regular expression denial of service (ReDoS).\n\nCalls to ``bleach.clean`` with an allowed tag with an allowed ``style`` attribute are vulnerable to ReDoS. For example, ``bleach.clean(..., attributes={'a': ['style']})``.\n\n### Patches\n\n3.1.4\n\n### Workarounds\n\n* do not whitelist the style attribute in `bleach.clean` calls\n\n* limit input string length\n\n### References\n\n* https://bugzilla.mozilla.org/show_bug.cgi?id=1623633\n* https://www.regular-expressions.info/redos.html\n* https://blog.r2c.dev/posts/finding-python-redos-bugs-at-scale-using-dlint-and-r2c/\n* https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6817\n\n### Credits\n\n* Reported by schwag09 of r2c\n\n### For more information\nIf you have any questions or comments about this advisory:\n\n* Open an issue at https://github.com/mozilla/bleach/issues\n* Email us at security@mozilla.org","published":"2023-02-16T22:15:10.650Z","modified":"2026-10-20T10:54:23.342990607Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":{"score":0.00713,"percentile":0.50543,"asOf":"2026-08-23"},"cisaKev":null,"exploitsKnown":1,"affectedPackages":[{"ecosystem":"PyPI","name":"bleach","fixedVersion":"3.1.4"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/mozilla/bleach/security/advisories/GHSA-vqhp-cxgc-6wmm"},{"type":"REPORT","url":"https://bugzilla.mozilla.org/show_bug.cgi?id=1623633"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2020-6817"},{"type":"PACKAGE","url":"https://github.com/mozilla/bleach"},{"type":"WEB","url":"https://github.com/mozilla/bleach/releases/tag/v3.1.4"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2020-340.yaml"},{"type":"WEB","url":"https://snyk.io/vuln/SNYK-PYTHON-BLEACH-561754"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-20T10:54:23.342990607Z"}}